Until now, a small and thin image pickup device has become mounted on a mobile terminal such as a cell phone and a PDA (Personal Digital Assistant). Thereby, not only sound information, but also image information can be mutually transmitted to/from a remote place. As an image pickup element for use in such the image pickup device, there are used CCD (Charge Coupled Device) type image sensor and CMOS (Complementary Metal-Oxide Semiconductor) type image sensor. In recent years, demands for adopting an auto-focusing function in these image pickup devices are increasing additionally to demands for furthermore downsizing them.
In order to achieve an auto-focusing function, for example in Patent Literature 1, an image pickup device for use in a mobile phone is configured to move an image pickup lens along the optical axis to adjust the focus to a subject. The image pickup device includes an image pickup lens composed of plural lenses, a lens holder for holding the image pickup lens, a rotary tube screwed with the lens holder, and a stepping motor for rotating the rotary tube. When the stepping motor rotates the rotary tube, the lens holder straightly moves in the optical axis direction and the image pickup lens moves on the optical axis up to a best focus position and stops. As described above, the auto-focusing function is performed.
However, in the above prior art, a rotary tube is arranged at the outer circumference side of the image pickup lens and a stepping motor is further arranged at the outside of the rotary tube, which makes the image pickup lens large in size in the radial direction. Further, in the prior art, lenses at the side of the image pickup element also straightly move in the optical axis direction together with the lens holder. Therefore, the air flows into a space around an image plane corresponding to a movement of lenses at the side of the image pickup element, and dust can adhere to an area around the image plane.
In Patent Literature 2, in order to downsize an image pickup device, the image pickup lens includes, in order form a subject, an aperture stop, a first lens with positive power, a second lens with negative power, a third with positive power, and a fourth lens with negative power, wherein a focusing operation is performed by moving only the first lens in the optical axis direction. Because the first lens is a lens with the smallest outside diameter, the image pickup device is not enlarged even when an actuator is arranged outside the first lens in the radial direction.
In recent years, as for an image pickup device, corresponding to an increase of the number of pixels of an image pickup element, demands of enhanced resolution on an image pickup lens is increasing, additionally to a demand of downsizing on an image pickup device. Especially, enhancement of optical properties such as a MTF (Modulation Transfer function) characteristic at image pickup at a close distance such as macro-photography, and an image pickup lens with a small Fno (excellent lens speed) are required. However, to meet the demands, the above prior art employs a focusing operation wherein only the first lens is moved, which significantly deteriorates the MTF characteristic at a large-image-height position on an image plane of the image pickup element (off-axis position), with respect to that at an axial position. Further, when performing a focusing operation, the image pickup lens can move and be displaced from the optical axis due to a drive mechanism of lenses. When a focusing lens is decentered, especially it is tilted with respect to the optical axis (tilt decentration), a focal point at a off-axis position is shifted in comparison with an axial position, and the MTF characteristic is deteriorated corresponding to the focusing operation. Especially in an image pickup lens with a small Fno, the MTF characteristic is significantly deteriorated. An influence of the tilt decentration on the optical properties such as the MTF characteristic (decentration sensitivity) becomes significantly great when a single lens is moved. Accordingly, because only the first lens is moved in the prior art, there was problems that the decentration sensitivity becomes large, optical properties is deteriorated and it is hardly applied to an image pickup lens with a small Fno.